This invention relates generally to image forming optical systems, and more particularly the invention relates to phase shifting test mask patterns for characterizing illumination and mask quality in such systems.
A phase shifting mask alters the phase of light passing through certain areas of a photo mask in order to utilize destructive interference to improve resolution and depth of focus in optical lithography. The semiconductor industry is now considering such masks for use in fabricating integrated circuits such as mega bit memory and microprocessors which contain millions of transistors.
The phase shifting mask and its use in an image forming optical system must be free of printable defects. To this end, a mask with phase shifted regions can act as a precision instrument for characterizing the imaging.
Copending application Ser. No. 11/053,725, filed Feb. 7, 2005 discloses three new classes of phase-shifting mask patterns. In optical image forming systems, a mask with phase shifted regions can act as a precision instrument for characterizing imaging. Three new classes of phase-shifting mask patterns have been invented to characterize projection printing tool illumination and phase-shifting mask (PSM) performance. The linear phase grating (LPG) and linear phase ring (LPR) both serve to characterize illumination angular distribution and uniformity. A third new class, the interferometric-probe monitor for phase-shifting masks (IPM-PSM), measures the effective phase, transmittance and edge effects of various phase-shifted mask features. All three patterns allow performance comparison across the field, tool-to-tool, over time, or to intended design.
The present invention is directed to a new class of phase shifting test mask to characterize both linear and circular polarization balance of the illumination incident upon the object in high numerical aperature (NA) and immersion projection printing system. The patterns are referred to as Phase Shift Mask Polarization Analyzers and exist in two embodiments: a radial phase grating (RPG) and a more sensitive Proximity Effect Polarization Analyzer (PEPA).